1. Field of the Invention
The invention relates to processes and apparatuses for purifying chlorosilanes by distillation and adsorption.
2. Description of the Related Art
In particular, the invention relates to a process for separating a multicomponent mixture comprising chlorosilanes into its components while simultaneously depleting the level of impurities (boron, phosphorus, arsenic) in the chlorosilane mixtures.
Chlorosilanes such as trichlorosilane (TCS) are used for depositing polycrystalline silicon.
TCS is primarily produced by three different processes:
A) Si+3 HCl→SiHCl3+H2+byproducts (hydrochlorination of metallurgical silicon)
B) Si+3 SiCl4+2H2→4SiHCl3+byproducts (reaction of metallurgical silicon with silicon tetrachloride/STC and hydrogen)
C) SiCl4+H2→SiHCl3+HCl+byproducts (hydrogenation of silicon tetrachloride/STC)
Byproducts generated include, inter alia, dichlorosilane (DCS).
It is preferable when a mixture of chlorosilanes comprising TCS, STC, DCS and traces of further impurities (methylchlorosilanes, hydrocarbons, high boilers) is concerned.
High-purity trichlorosilane is obtained by carrying out a subsequent distillation in each case. An essential object of the distillation is the removal of boron-, phosphorus- and arsenic-containing compounds since said compounds are unwanted p-/n-dopants in the deposited silicon. In respect of these impurities the purity requirements for trichlorosilane employed in deposition are in the range of just a few ppta. Distillative processes are customary in chemical engineering to thermally separate mixtures of different relative volatility and/or mutually soluble substances.
Various process versions are commonly used for continuous distillative resolution of multisubstance mixtures.
In the simplest case a feed mixture composed of a low-boiling fraction and a high-boiling fraction is resolved into its two fractions, a low-boiling tops fraction and a high-boiling bottoms fraction. Here, the mixture to be separated is introduced between the bottom and the top of the distillation column. The feed divides the column into a rectifying section and a stripping section. The high-boiling fraction is withdrawn from the column in the bottoms. A portion of the concentrate is evaporated by a heating means (e.g. a natural circulation evaporator) incorporated in the bottom region. The low boiler ascends the column as vapor, is withdrawn from the column at the top of said column and is liquefied in a condenser. A portion of the condensate is recycled into the column again and runs downward in countercurrent to the ascending vapors (reflux).
However, the fractionation into more than two fractions of feed mixtures composed of a multicomponent mixture (A, B, C) then requires the use of a plurality of conventional distillation columns. There are several options to achieve this.
For the a-path the low boiler A is removed as a tops product in a first column. The bottoms fraction is a mixture of middle boiler B and high boiler C which is fractionated in a downstream column into the two pure substances B and C.
For material coupling with preseparation (a/c-path) the separation in the first column is performed such that the tops product comprises no high boiler C and the bottoms product comprises no low boiler A. The separation effected is thus that of the low boiler A and the high boiler C. The middle boiler B is present both in the tops fraction and in the bottoms fraction. Both fractions AB and BC are resolved, each in a separate downstream column, into the pure products A, B, and C. This version thus requires three separation steps.
For the c-path C is removed in the first column as pure bottoms product and mixture AB is transferred to the second column as tops product, typically in vaporous form.
It is generally the case that for fractionation of a three-component mixture the choice of a suitable path (a-path, c-path, a/c-path) depends on the composition of the input.
For high contents of low boiler A the a-path is preferred. By contrast the c-path is preferred for high contents of high boiler C.
When the middle boiler proportion B is high it is preferable to choose the a/c-path. For material coupling with a precolumn both columns are materially coupled (thus two-fold material coupling; so-called Petlyuk setup).
US 20120193214 A1 discloses a process for distillative purification of chlorosilanes which comprises providing a boron-containing mixture of chlorosilanes comprising TCS, DCS and STC and purifying the mixture of chlorosilanes by distillation in a plurality of distillation columns, wherein low-boiling boron compounds are tapped off from the distillation columns via tops streams comprising boron-enriched DCS and relatively high-boiling boron compounds are tapped off from the distillation columns via a boron-enriched bottoms stream comprising high boilers.
In addition to purely distillative processes it is also known to employ adsorbers.
The adsorber can fulfill various functions. Trace compounds may be retained from the trichlorosilane by adsorption mechanisms. This is an effective method of removal especially for polar molecules. The adsorbent may further be conditioned in order that a chemical reaction to convert these compounds begins on its surface. For instance deliberate hydrolysis on water-conditioned adsorber surfaces is a known method of generating boron-oxygen compounds that are markedly easier to remove in downstream distillation steps, see U.S. Pat. No. 4,713,230 A for example.
US 20110184205 A1 describes a process for treating a composition comprising at least one silicon compound and at least one extraneous metal and/or an extraneous metal-comprising compound, wherein the composition is contacted with at least one adsorption medium and/or at least one first filter in a first step and optionally contacted with at least one filter in a further step to obtain a composition having a reduced content of the extraneous metal and/or the extraneous metal-comprising compound.
Here, the boron content in chlorosilanes is reduced by contacting with water-free adsorber media (activated carbon, silicates such as silica gel, zeolites, organic resins). However, very large amounts of adsorber medium (120 g/250 ml of TCS) are required to achieve the desired purification objective. This makes the process uneconomical, especially as a continuous process is hardly possible, which is economically disadvantageous in the production of semiconductor-quality chlorosilanes. The use of adsorbers moreover necessitates further apparatus complexity (such as filtration) and brings with it the risk of introducing other impurities in the semiconductor-grade product.
US 20130121907 A1 discloses a process for removing at least one boron-containing impurity from a mixture comprising trichlorosilane to afford a purified product comprising trichlorosilane. This comprises partially removing the boron-containing impurity from the mixture to obtain a partially purified mixture comprising trichlorosilane. This partially purified mixture is supplied to a column through a side feed port. Discharged from the column are
a) a tops product comprising a boron-containing impurity
b) a bottoms product comprising a boron-containing impurity
c) and a purified mixture comprising trichlorosilane.
The partial removal of the boron-containing impurity may comprise supplying the mixture to a distillation column, discharging a tops product comprising boron-containing impurities and withdrawing a partially purified mixture as bottoms product. This bottoms product may be passed through an adsorber, for example a silica-gel bed, before it is supplied to the second column.
Despite a combination of adsorbers and distillation columns the distillation steps in the production of semiconductor-quality trichlorosilane still have enormous energy requirements. One attractive option for reducing energy requirements is offered by dividing wall column technology. This technology is based on the principle of material coupling and allows for a reduction in energy requirements of up to 50%. Since this process takes on the separation task of two apparatuses it is thus also possible to economize on capital expenditure.
Conventional dividing wall columns have a vertical dividing wall disposed in the column longitudinal direction which prevents transverse mixing of liquid and vapor streams in subregions of the column. This column thus comprises at least one vertical dividing wall which runs along part of the column height and divides the cross section into at least two segments to the left and right of the dividing wall.
It is thus possible to resolve, for example, a three component mixture into its three pure constituents in a single column which would normally require two conventional columns.
The dividing wall disposed in the column longitudinal direction separates the column interior into a feed section, a withdrawal section, an upper common column section (rectifying section) and a lower common column section (stripping section).
However combining the dividing wall column with the use of adsorbers known hitherto has proven costly and inconvenient to implement. The material coupling in the dividing wall column has the result that two separation tasks are performed in one apparatus. Both low-boiling boron compounds and high-boiling boron compounds are removed in the apparatus. Positioning the adsorber in the feed to the column is not advisable since the content of boron compounds at this point is still very high which would lead to very rapid loading of the adsorber. To achieve the same effect of the adsorber in a conventional dividing wall column would require providing said column with corresponding internals.
It is known to employ catalytically active internals in dividing wall columns. EP1513791 B1 discloses a distillation column having at least two vertical distillation segments, wherein at least one of the segments comprises catalyst and at least one of the segments is free of catalyst, wherein the segments are divided by a wall extending along a vertical portion of the distillation column, wherein the vertical portion comprises less than the total height of the column and the segments are in fluid communication around a vertical terminus/end of the wall.
A similar concept would in principle also be conceivable for the use of adsorbers in a dividing wall column. However, since the adsorbers need to be replaced at regular intervals (loading, conditioning) this version is disadvantageous. Since the distillation is a continuous process, relatively long interruptions in operation due to regularly required replacement of the internals are not desired.
The object to be achieved by the invention arose from the problems described above.